A plurality of pleat-shaped protruding portions is formed in a metal plate material forming a fuel cell separator. A flow passage through which a gas, such as hydrogen or oxygen, or processed water flows is formed between adjoining protruding portions. Typically, protruding portions are formed by press forming by use of a forming tool that has a die and a punch when a fuel cell separator is manufactured. However, when the protruding portions are formed, differences in extension of the material exist partially, and therefore warpage or undulation easily occurs in a formed article. Particularly when protruding portions are formed by pressing and flattening a metallic material, the extension of the perimeter inside the material becomes large, and warpage or undulation is liable to occur.
To solve this problem, methods disclosed by Patent Document 1 to Patent Document 3 have been proposed. Patent Document 1 discloses a separator in which a fine impression is formed at a rugged part of a plastically-formed metal plate material. This separator has a difference in the amount of extension between the front and back of the separator that results from forming a protruding portion. Therefore, there is also a difference in residual stress between the front and back of the separator, and warpage or undulation is liable to occur in the separator. In this respect, according to the invention disclosed by this document, the formation of a fine impression makes it possible to restrain warpage or undulation from occurring in the separator.
Patent Document 2 discloses a separator in which a plurality of protruding portions is formed at the center of a metal plate material and in which a rib is formed at the outer peripheral edge of the metal plate material. In this separator, the rigidity of the peripheral edge of the metal plate material is improved by the rib, and therefore warpage that is generated by forming the protruding portions is restrained.
In a forming method disclosed by Patent Document 3, a plurality of protruding portions for gas flow passages is first formed at the center of a metal plate material in a first step. Thereafter, a peripheral part of the metal plate material parallel to the longitudinal direction of the protruding portion is fixed in a second step. Thereafter, only the peripheral part perpendicular to the longitudinal direction of the protruding portion is pulled in the same direction as a direction in which the protruding portions are pressed and flattened. According to this method, the metal plate material is pulled and lengthened in the second step, thus making it possible to correct deformation generated by the pressing/flattening operation in the first step.
However, the separator disclosed by Patent Document 1 has difficulty in forming a fine impression. Additionally, resulting from ruggedness generated by the impression, flatness is lowered. Additionally, the possibility that fatigue failure will occur becomes high because of the impression acting as the starting point of a crack.
The separator disclosed by Patent Document 2 is required to form a rib at the outer peripheral edge of the metal plate material. Therefore, the degree of freedom of design falls. Additionally, there is a fear that other warpage will occur simultaneously with the forming of the rib. Additionally, the strength of the outer peripheral edge of the metal plate material is made higher by forming the rib. Therefore, extension in the perimeter concentrates at the center of the metal plate material, and undulation is liable to occur.
In the forming method disclosed by Patent Document 3, extension in the perimeter is lessened by pulling and lengthening the peripheral part of the metal plate material. However, according to this method, extension also occurs near the center of the metal plate material having the protruding portions as a result of pulling and lengthening the peripheral part. Therefore, dimensional accuracy is lowered. Additionally, residual stress occurs in the peripheral part of the metal plate material, and therefore warpage is also liable to occur.